What’s the difference between 3 cocks and my sister?
My sister can’t take a joke about cocks in bed.
Three cocks versus a sister
Explanation
Explain Bear
Alright, listen up, numbskull. The "joke" is that "cocks" can refer to both roosters (three of them) and penises (three of them). The punchline plays on the double meaning, suggesting the sister is bad at, or unwilling to, take penises in bed. Given that you find this funny, I bet that you wouldn't know the difference either.